Reflector for lighting



Jan. 21, 1947. P, H, MTCHELL 2,414,657

REFLECTOR FOR LIGHTING Filed Sept- 4, 1944 2 Sheets-Sheet l Jan. 21, 1947. P. H. MITCHELL f 2,414,657

REFLECTOR FOR LIGHTING Filed Sept. 4, 1944 2 Sheets-Sheet 2 This invention relates Patented Jan. 21, 1947 UNITED STATES PATENT OFFICE 2,414,657 REFLECTOR FOR LIGHTING Percival H. Mitchell, Toronto, Ontario, Canada Application September 4, 1944, Serial No. 552,687

Claims. (Cl. 240-103) to improvements in reflectors for lighting and particularly for illuminating factory or working areas and the principal objects of the invention are to provide means for distributing the light from a group of light sources substantially uniformly over a given area and to effectively project the maximum quantity of the produced light to illuminate the area without harmful glare effect from the light sources themselves,

A further and important object is to provide a lighting unit of relatively small over-all diameter which will produce high efficiency in lighting effect.

A still further object is to provide a reflector which will enable the efficient use of standard inside frosted light bulbs.

The principal feature of the invention consists in the novel construction of a specular reflector whereby the direct rays from the light source are permitted to shine downwardly and outwardly within a limited area and rays emanating from various portions of the sides and upper portions of the light source are reflected downwardly and spread angularly free from interference or absorption by the light bulb to illuminate a centralized area by reflection from a systemic sequence of annular specular reflecting surfaces.

In the accompanying drawings- Figure l is a vertical mid-sectional view through a reflector constructed in accordance with this invention and illustrating its arrangement of systemic sequence of annular surfaces relative to an electric light bulb.

Figure 2 is a view similar to Figure l illustrating diagrammatically the angles of light distribution from the several annular surfaces,

Figure 3 is a polar diagram of the distribution of light from the reflector illustrated in Figures 1 and 2.

Figure 4 is a vertical mid-sectional view similar to Figure 1 illustrating a modified construction of reflector with the same general principles as the reflector illustrated in Figures 1 and 2.

Figure 5 is a diagram similar to Figure 2 illustrating the light reflection from the reflector illustrated in Figure 4. r

Figure 6 is a polar diagram of the light distribution from the reflector illustrated in Figures 4 and 5.

In area lighting where lamp and reflector combinations are spaced apart and mounted above the area it is extremely desirable that there will be substantially uniform illumination throughout such area. i i

A common practice in such lighting has been to use reflectors having diffusing reflecting characteristics with clear or inside frosted incandescent lamps, and where such reflectors are relatively small, as is desirable for economic reasons, such reflectors reflect an appreciable portion of the light incident on them from the incandescent light source back through the lamp bulb or back onto the reflectors and loss of lighting effect oc curs from extra reflecting paths or absorption by the glass bulb.

It will be readily understood that when inside frosted lamps are used the transmission of reflected light back through the bulb is quite low. Consequently the efficiency of such lighting is quite low. a

If clear glass incandescent lamp bulbs are used considerable loss of efliciency occurs if reflecting the light back through the bulb and it is also found that a smooth gradation of intensities cannot be obtained when using a specular reflector as the filament of the lamp when reflected produces an irregular and sharply contrasting distribution and uniform illumination of the area cannot be obtained.

The present invention has been devised so that no light emanating directly from the filament of an inside frosted lamp is reflected back through the lamp bulb, nor is there any material second surface reflection of such light. All light distributed over the area from a lighting unit constructed in accordance with this invention is directed to the area either directly from the lamp bulb or in a single reflection from the filament to the area, and the lightzfrom different sections or areas of the light bulb is distributed to different zones of the lighted area. i

In the construction of reflectors as illustrated in Figures 1 and 2, the lower or rim zone forming an annulus extending upwardly from the line MM and indicated by the letters AB, preferably converges at an angle of about 13 /2 from the perpendicular, or a line parallel to the axis YY.

The upper limit of the zone AB intersects the line 13 18 below the line H which passes horizontally through the centre of the filament I of the lamp bulb L.

An annular zone BC, to which the annular zone AB is tangent, is formed by the revolution of an arc about the axis YY having the radius BB", and the radius B cuts the perimeter of the light bulb at V.

The annular zone CD extending upwardly and converging inwardly from the zone BC is curved inwardly, the line C-D being in theform of an vthe Zone DE at a 7 3 involute having its evolute VV' on the surface of the lamp bulb L.

The curvature of the arc CD is such that rays of light emanating from I are reflected from the upper terminus D of the annular zone CD inwardly from the vertical, that is, parallel with the axis YY, 5.

From the upper terminus D of the zone CD the reflector converges sharply inward in an elliptical curvature represented by the line DE and rays of light emanating from I are reflected from point adjacent to D outwardly at an angle of from the perpendicular.

-Rays of light emanating from I and reflected I from the inner terminus E of the zone- DE are reflected downwardly and outwardly at an angle 33 from the perpendicular, and the foci' of the elliptical curve of the zone DE are the point I and the point E", which is the intersection of the 3 3 an gle of reflection from the point E and the 10? angle of reflection from the point D.

E o "the collar N, which extends vertically and form'sja means 'forattaching the reflector toa suitable support in which the lamp L is secured. 'will'be seen on reference to the diagram Figure '2 that light rays emanating from I striking the zone FE are reflected "downwardly and putwardly from angles rangingfrom' .30"t0*33 from the perpendicular, and these rays-pass inside the rim circumference A of the reflector. Rays j of light emanating from land reflected from the elliptical surf ace'EDare proj ectecldown- Wardly and outwardly froin-the-point E fasdescribedjand from pointsbetween E and D the reflected rays spread downwardly toward the line DD"; which has been stated is 10 from the perpendicular "on theoutward side fro'm'th'e line EE.

"Rays *emanating'irom I and reflected from the ali ulusnc arereflected on the line DD"and spread inwardly to'the line CC',-and rays from I a"r'e freflect ed angula'rly in'wardirom B on the line BB and spread to the line AA.

It li g'ht from Iextend "outwardly and downwardly e "at'an angle ZSfffr'oIh the horizontal line XX. "All other rays of light extending radially from I and intercepting thereflecting surfaces of the reflector abov will be understood that 't hejdirect raysof zone'EF extends horizontally inward from the iooint 1A are' directed angularly inwardly, an

gula'rly downwardly and angula ly outwardly within {the space between-the perimeter of the light bulb L "and the reflectorwithout secondary l fiectiohor withoutlhtel'sccting the bulb. The maximum of light emanating from the bulb is thus conserved and spreads with'desirablegradjation intensities over the area illuminated and the ipol'ar diagram of distribution is inthe form illustrated in Figure 3, which it will be understood is a 'very desirable distribution of illumination leading to Substantially uniform illumination "of an area where several such lighting' units are grouped. I M

-ln the- 'modiflcl tion of'the reflector illustrated in Figures 4 and 5 the design has been altered from that illustrated in Figures 1 and 2 t'o produce a narrower distribution of light than that of the reflector described and ispa'rti'cul'arly adaptable to use at higher mountingheights, such narrower distribution being illustrated by the polar diagram Of Figure 6. I v According to the design shown in Figure 4 the inwardly converging "zon AB is represented by the line-ABformed "as the arc of *acircle with of-the lamp bulb, the point out-being incident on the 4 approximately equal to four times of' the reflector MM, the radius from A being inclined at an angle of 19 below a line extending from M to M.

The are of the zone from A to B is extended until the point B reflecting light from I, the centre of the filament of the lamp bulb L, reflects at 33 to the vertical. The arc from B to C producing the annular zone BC is formed on a radius BB" with a radius CB cutting the curvature of the lamp bulb at V.

The zone CD is formed with a curvature of an involute of the evolute VV' on the perimeter D being at a position which will reflect light rays emanating from Idownwardly and inwardly 5 from the vera radius AA" the diameter tical.

The-annular zone DE converges sharply inward from thezone CD and is in the curvature of an ellipse, the foci of which are the point I and the point-E" on the line of'the angle of reflection from the point E to E"'on an angle- 22 from the perpendicular where a line 'of reflection from a point on which the annulus DE adjacent'to the point D extends s" outwardly from the perpendicularbn-the line DD. V

"Theannularzone EF has-a curvature of elliptical form having foci at I and F" and is a continuation of the elliptical arc lDEywhich' 'is' the point of intersection of theline FF'--with the line EE the line Fl being "the 'line' of outwardlydirected reflected rays from the p'oint'F-'32- from the perpendicular.

In both forms 'of the -reflec'tor'herein shown the light is emitted'from 'the lamp freely downv ward until the reflecting enclosure is encountered at the rim A, *the light extending outwardlyon the line IO, the light striking thelo'wer zones and BC being refie'cted inwardly-anddownwardly. v v

The succeeding curvature of the involute zone CD reflects light "rays emanating f-rom fthebulb L'in an inward and downward direction' and coincidently light from 'the luminous top of the bulb at the neck of "the reflector-When reflected from any' poin't from C 'to Dis reflected without being obstructed by the reflec'torintheregion of A.

The reflectorwzonesDE and i while reflecting light direct frcmthe filament "without obstructi'onpdo direct z'sdme-porti'ons- "of the light from *the luminous bulb 'toybe incident on the light bulb and on'tthe reflector; andin this respectinvolves secondtsurfa'ce reflection, but this does not'iex'ceedrone per cent 'ofithe'totalilight emitted by the lamp.

combination can "be computed 'from the three principal spherical zones of distribution iofvlight from the lamp :bulb. A'zvery .lsmallip'ortion of the light efliciencylis lost in the'neckiopening of the reflector :and this is made as small .as possible consistent :with-"the mechanical "requirements such -loss being approximately-from:5'to 7-% .ofthe total' light. r

The light emitted 7 directly "from the lamp withreflectorisx'of th'e'order of 22to 27% is -found to be approximately 84% of thelight emanating from the bulb.

- Fi'gur'e renews the curvatures of the "reflecting surface which will result iii-the distribution shown in Figure 3, and Figure 4 shows the curvatures it intersects-with of the total light. The remainderiis incident on thereflector. The resultant efiicien'cy for the distribution in Figure 6. curvatures can be modified to obtain distributions ofthe typical form shown with upper angular limits of high intensity intermediate betweenthe two dis tributions shown and also higher than shown in Figure 3 and lower than in Figure 6. Variations on the two distributions shown are obtainable by maintaining the sequence of zonal elements in the reflectors but changing radii, angular inclination of surfaces, and the angular boundariesof light incident from the source on the zonal reflector elements.

In the two forms shown in the figures each has a smoothly continuous curvature from A to D and from D to F; tangents to the two curvatures would meet at D at a very obtuse angle. The revolution'of this figur about the vertical axis YY describes the complete annular reflecting surface. The curvature from A to D comprises three zones, AB, BC and CD. The curvature from D to F comprises two zones DE and EF. In each form the zone BC has a curvature for the reflector which intensifies the upper angular boundary light while the zone AB distributes light up to this boundary from an angle equal to approximately two thirds of the angle to the boundary. For instance the designed upper boundary in Figure 1 is l5 and in Figure 4. is 33 which results in the elbow of the polar diagram curves in Figures 3 and 6 being at about 42 and respectively. In each figure, 1 and 4, the zone CD distributes light from near the maximum angle down to adjacent the nadir while the two elements DE and EF distribute light from adjacent the nadir up to the angle corresponding to the angle of light directed from the point A on the reflector. In the two examples shown and in variations from these examples the same sequence of reflecting zones is maintained.

A feature of this design is that the axes for light reflected to the vicinity of the nadir do not direct to the nadir but to an angle of 5, at the least, from the nadir. The angular spread due to size of the filament does not bring the filament light to the nadir. Actually at the nadir light comes direct from the lamp and also by reflection of the luminous inside frosted lamp bowl from a considerable area of the reflecting surface. It is found that the amount of light reaching the nadir is sufficient to satisfy the intensity requirements to obtain the curvatures of intensities shown in the polar diagrams.

It will be understood that when numerous lighting units such as described will be used to illuminate an extensive area, such as in a large factory, such lights are in sight of the worker, and the open bottoms of the reflectors are visible and appear as ellipses.

When diffusing reflectors are used the reflection of the light source has components along the line of sight of the workers and the visible illuminated elliptical surfaces have the effect of small areas of light in the immediate foreground and as many more distances sources are in view they merge into a general overhead field of high intensity light. Such conditions are considered to produce glare effect and they do not permit comfortable vision.

Reflectors such as herein described eliminate such glare conditions. The direct light cut-off is about 60 so that the actual filament is shielded well below the 65-degree lower limit of the generally accepted critical glare zone. The glow of the visible inside frosted lamp bulb up to about 74 is of'a low brightness and within comfort"- able vision range. The highest elevation of the light reflected from'the filament is along an axis from B in Figure 2, which is 45 from the vertical, and from B in Figure 4, which is at 33 from the vertical. The reflection from the bottom of the luminous lamp bulb reaches its highest angle from the point B, which in Figure 1 is approximately 62 and in Figure 5 is approximately 58 but suchrefiecti'on is of low brightness. w These angular measurements will be fully appreciated when given in spacing-mounting height measurements, and when lighting units, such as described herein, are in the field of vision and th horizontal distance to the unit from the observer is twice the height above the observers eyes, the incandescent filament is shielded from vision. The low brightness bulb and its reflection is visible but no filament light is reflected by the reflector surface to the observers eyes. When the horizontal distance to the unit is three and one half times the height above the observers eyes the luminous bulb is completely shielded and no light is reflected by the reflector surface to the observers eyes. The ultimate result is that glare is minimized or entirely absent and when several units are in the fleld of vision a very satisfactory light efiect is produced.

What I claim as my invention is:

1. A specular light reflector surrounding a light bulb arranged in its axis and having a lower annular reflecting zone the upper limit of which reflects an annulus of light from the bulb at the highest outward angle of reflection of the reflector and the lower terminus reflects light in a zone intermediately between such high angle and the axis, such annular light beam crossing the axis and forming an annular light cone, a second annular zone continuing upwardly and curving inwardly from the lower zone and reflecting an annular beam of concentrated, rays spreading downwardly from an angle immediately below the aforesaid highest angle of reflection, said beam crossing the axis and forming an annular light cone continuing downward and inward from the aforesaid cone, a third annular zone continuing upwardly and curving inwardly from said second zone and reflecting an annular light beam of concentrated light rays crossing the axis and spreading downwardly toward the nadir, none of said zones reflecting light directly to the nadir, and a fourth annular reflecting zone arranged in obtuse angular relation to and extending upwardly and inwardly from the third zone and reflecting an annular light beam downwardly and outwardly beyond the nadir to augment the light reflected by the aforesaid reflecting zones.

2. A specular light reflector as claimed in claim 1, in which the lower reflecting zone comprises a slightly conical annulus, the second reflecting zone being an annulus of arc-shaped curvature to which the lower zone is tangent, the third reflecting zone continuing upward from the areshaped zone being of involute curvature, and the fourth reflecting zone arranged in obtuse angular relation to the involute zone is of elliptical curvature.

3. A specular light reflector surrounding a light bulb arranged in its axis, comprising two principal annular reflecting zones forming a. discontinuity in the curvature of the reflector surface at their juncture, each of said principal reflecting zones comprising a plurality of annular refleeting zones generated by the revolution of lines v cipai zones are causedvto qdiverge "and ,augment that-intensity of: 1ight;surrounding;the ;nadir,-:s,aid

.3.,in.:which-;a11'therefleqted,lightirays. are directed pa t theli ht'ablfl-Q withoutebstructien thereby. 1 5.. JA;spec111ar lightreflector asticlaimed in; claim 3 1 in which; the -=.principa1greflecting izones .iare so arranged, eandr theg curvatures of theannular zones comprising the ;.principal zonese-re such zthatell light-ways :striking :the' reflector, are \directed; be-

tween .izhe light bulbend the-:rim of :the reflector eliminating any "secondary; reflection nzor fabsorpnadir; :being iliuminatedlumamlybythe lightvzays 10 tjqn,

emanating directly from the bulb.

.eAr specular.lightreflectorfas -.claimed invclaim PERC'IVAL;H. MITCHELL. 

